1. Field of the Invention
The present invention relates to mass spectrometry.
2. Background Information
Mass spectrometers can be used to determine the existence of trace molecules in a gas sample. FIG. 1 shows a quadrupole mass spectrometer which contains an electron-ionizer 1. The electron-ionizer 1 includes a filament 2 that extends around an anode grid cage 3. A gas sample is introduced into an ionization chamber 4 of the ionizer 1. The filament 2 bombards the gas sample with electrons to ionize molecules within the sample.
The spectrometer also includes a mass analyzer 5 which can determine the mass of the ionized molecules. The anode grid cage 3 is typically provided with a positive voltage potential to accelerate the ionized molecules into the mass analyzer 5. The mass analyzer 5 may contain an entrance plate 6 which has a negative voltage potential and two pairs of quadrupole rods 7 that are at an average potential near ground to pull the ionized molecules into the analyzer 5. The electron-ionizer 1 may also have a repeller cage 8 to contain the ionized molecules within the ionization chamber 4. The mass analyzer 5 provides output signals that are a function of the mass of the molecules detected by the analyzer.
It has been found that electron-ionization may create fragmentation which increases the number of different ions that are detected by the analyzer. The greater number of different ions formed increases the number of output signals detected by the analyzer. The additional output signals may result in erroneous conclusions regarding the content of the gas sample, particularly if there are two or more ionized molecules with approximately the same weight.
U.S. Pat. No. 5,808,299 issued to Syage discloses a mass spectrometer which contains a photoionizer. The photoionizer includes a light source which directs a light beam into a gas sample. The light beam contains energy which is high enough to ionize the trace molecules but below the energy level which typically causes fragmentation. Photoionization can therefore provide more reliable data from the mass spectrometer. It would be desirable to have an electron-ionization mass spectrometer that can photoionize a gas sample. It would also be desirable to modify an existing electron-ionization mass spectrometer to include a photoionizer.
There are also mass spectrometers which utilize chemical ionization wherein an electron or a proton is attached to the trace molecules. Chemical ionization may be achieved at "atmospheric" pressure. Atmospheric ionization pressure being a pressure level that is higher than the vacuum pressure of the mass detector of the spectrometer. Higher ionization pressure levels increases the density of the gas sample. The higher gas sample density increases the number of ionized trace molecules and the sensitivity of the mass spectrometer.
Chemical ionization can be effective when detecting trace molecules which have high electron or proton affinity. The detection of molecules that do not have a strong electron or proton affinity can be compromised when other molecules are present which do have a high affinity. For example, water is an abundant molecule which has a high proton affinity which competes for positive charges. Even if sufficient charge exists in the ionization source to ionize weakly interacting low abundance molecules, the presence of a strong protonated water H.sub.3 O.sup.+ signal can overwhelm the detection of very weak signals from trace molecules of interest. Likewise for negative ion detection by electron attachment, oxygen molecules compete with trace molecules for electrons thereby reducing the number of ionized trace molecules and the sensitivity of the mass spectrometer. It would be desirable to provide an ionizer which ionizes a gas sample at atmospheric pressure but does not have the unfavorable characteristics of chemical ionization.